Yahoo! has launched a new application that lets you keep track of trends and search terms in Twitter from your desktop. While Yahoo! Sideline is based on Adobe AIR, it’s not a full fledged desktop client for Twitter like Twhirl or Tweetdeck.It doesn’t let you send messages or keep track of tweets from your friends. Instead, Sideline lets you enter search terms and see messages from all Twitter users using those terms.

You can also create Search Groups with multiple terms. For instance, say you wanted to track everything said about Download Squad. You could create a group called Download Squad and then enter “download squad,” “downloadsquad,” “bradlinder,” “leematthews,” “film_girl,” “grobertson,” “strutting” and other terms. Sideline displays messages matching all of those terms and refreshes the search at regular intervals.

There’s also a Trends area where you can see topics that are popular across the Twitter network at the moment.

Overall, Sideline is kind of cool, but it doesn’t feel like something that needs to be a desktop application. You can create the same searches using Twitter’s Advanced Search tool. I suppose if you really, really want real-time notifications alerting you whenever someone mentions your name, your web site’s name, or a dozen other search terms, Sideline can come in handy. But for the rest of us, a normal Twitter client that lets you send and receive messages seems more useful.

If you’ve been using TweetDeck without a dual-display setup, it’s just about time for you to make a purchase. With newly-announced support for Facebook, you’re going to need some more real estate. The release comes two short days after Twhirl frontman Loic Le Meur announced that Seesmic had released a standalone Facebook app which, like TweetDeck, is built on Adobe Air.

Setup involves logging in to Facebook and the usual two or three clicks afterwards. Once completed, you’ll have a new panel showing your Facebook friends’ updates and a checkbox next to the update field to enable posting to Facebook. Curiously, TweetDeck includes the ability to email or tweet Facebook updates, something which doesn’t necessarily jive with Facebook’s TOS.

For now, that’s about it for functionality. You can’t, for example, send replies. With TweetDeck’s history of Twitter kung-fu there will likely be plenty of enhancements in the coming months.

There are quite a few popular Twitter-based applications around the web. But which ones are the most popular? TechCrunch put together a list of the Top 21 Twitter Applications (According to Compete). They looked at unique monthly visitor data from Compete for the month of January to come up with their list. Here are the first ten:

Twitter Applications

Monthly unique visits (Compete)

1. Twitpic

1,236,828

2. Tweetdeck

285,864

3. Digsby

233,472

4. Twitterfeed

149,812

5. Twitterholic

147,164

6. Twhirl

143,333

7. Twitturly

88,793

8. Twtpoll

74,154

9. Retweetist

60,051

10. Tweepler

51,304

There’s at least one problem with this list. Twellow is not on it (and yes, with regards to full disclosure, we do own Twellow), yet going by compete numbers, Twellow should have been number 7 at 107,866 unique monthly visitors.

If you are unfamiliar with Twellow, think about it as a yellow-pages service for finding people on Twitter. You can search for things or browse the directory to find people who tweet about the topics you are interested in. You can also use TwellowHood to find people by Geographic location. Stay updated on Twellow Features via the Twellow Blog.

Interestingly enough, the Compete data for Twellow is listed in TechCruch’s own CrunchBase. I’ve contacted TechCrunch asking why Twellow was left off the list, but have not yet received a response.

I don’t want to sound too nagging, but Twellow’s a top 7 Twitter App, and it’s not listed in the top 21, and we know the influence TechCrunch has in the industry. We just want people to be aware of Twellow’s place in the Twitter world.

We’ve accumulated a list of the twenty most popular Twitter applications, based on monthly unique visitor data from Compete.

Twitpic, an app that lets users share photos on Twitter, took the top spot with 1,236,828 unique visitors in January. Tweetdeck, which came in second with 285,864 monthly visits, is a Twitter app that streamlines notifications and tweets. Third place went to Digsby (with 233,472 monthly visitors), an application that centralizes e-mail, IM and social networking accounts into one desktop program. With 149,812 visits, the fourth most popular app, Twitterfeed, offers to automatically tweet posts published on a user’s blog using RSS. A Twitter user ranking site, Twitterholic, is the fifth most popular application, with 147,164 people visiting monthly.

Interestingly, two of the top twenty apps – Digsby and Hellotxt, an application that allows users to update their status across social networks – are not exclusively focused on Twitter. We recently wrote a detailed post on why we cover Twitter-related news so often, and we think the fact that there are so many applications developed as an offshoot of the microblogging system is another testament to the fact that Twitter is not just another startup. This is our attempt to figure out which ones are gaining the most traction.

While we think this is a pretty comprehensive list based on the chosen proxy for popularity, it’s but a first pass at a list we hope will become more authoritative over time. Keep in mind that this methodology is only one measurement of engagement; many people use Twitter iPhone apps and other clients without ever visiting these services’ websites. Derivative site traffic is only a proxy for usage but the most easily measurable one.

We also ran these sites though Quantcast and Google Trends, but chose to measure by Compete because it had the most comprehensive set of data, especially for the smaller sites. So the absolute numbers are unquestionably higher for some of these Twitter apps. If you know additional Twitter apps that should be listed here, or if you have better ideas on how to compile this list, please leave a message in the comments.

Update1: As many people are pointing out in the comments, Twhirl is not included in this list. That’s because the list is based on Compete data, and Compete measures Twhirl at under 13,000 visitors per month. As we said above, this is just the first pass at creating a list and we certainly realize Compete traffic is not a perfect way to gauge the popularity of applications.

Update 2: So it looks like we owe a “mea culpa” on Twhirl, since we looked up the wrong address on Compete originally (it’s at .org not .com). Twhirl.org actually has 143,000 visitors, putting it at the #6 spot.